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Sea slugs can’t remember their dreams — and here’s why you can’t, either

Scientists identified a seemingly counterintuitive process in the brain that prevents stimuli from forming memories. This system also springs into action as you are waking up to prevent corruption of previous memories — which might explain why it’s so hard to remember what you dreamed about.

Image credits Karolina / Pexels.

Without any prior training or a handbook handy when you wake up, it’s incredibly hard (and frustrating) to try and remember what you dreamed about a few moments before. It’s like you’re grasping at a shape in the fog — you know something was there and you have a rough idea of what it was like but every time you reach out you’re met with a handful of nothing. But why is it so hard to remember?

It all comes down to how our brain forms memories. Some of the stimuli that bombards you each and every day are deemed important enough to be memorized, which our neurons do by forming connections between each other — known as “trace memories”. This, however, is only a temporary measure, since these initial connections (and so the memories they maintain) are pretty fragile. To turn them into long-term memories, the brain has to go through a process called consolidation.

This involves synthesizing proteins to strengthen trace memories. However, if new stimuli are recorded while this process takes place, they could disrupt the process or overwrite the memory trace. So you might run into all sorts of problems if your brain started consolidating willy-nilly in the middle of the day. Thankfully, it evolved to only do so at night while you’re sleeping. But just in case you wake up during consolidation, the brain has mechanisms in place to prevent you from interrupting the process.

Slugging it out for memory space

A new study by Prof. Abraham Susswein of the Mina and Everard Goodman Faculty of Life Sciences and The Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center at Bar-Ilan University has identified this mechanism. He and his colleagues studied the sea hare Aplysia who are surprisingly convenient subjects for neuroscientific studies — they have simple nervous systems made up of large neurons, and have also shown basic learning abilities.

They found that after training the slugs’ brains started producing low levels of consolidating proteins, levels which spiked when the sea slugs went to sleep. But by blocking the production of these proteins in sleeping slugs, they were able to prevent them from forming long-term memories — confirming that they too consolidate memories during sleep.

They also found that exposing the animals to stimuli as they were waking up didn’t trigger the formation of new memories — they tried training the animals after awakening them from sleep, but the slugs couldn’t learn. On awakening, their brain blocked any interaction between the stimuli and long-term memory. When treating the slugs with a drug that inhibits protein production prior to training, the slugs could generate long-term memories however.

Removing the protein block allows the formation of long-term memories of experience just after waking up — even experiences that are too brief to trigger memories in fully awake slugs.

“The major insight from this research is that there is an active process in the brain which inhibits the ability to learn new things and protects the consolidation of memories,” Susswein says.

The team also found that training sea slugs in social isolation seems to inhibit their learning abilities, and identified a similar process active in this state.

“Our next step following on from this work is to identify these memory blocking proteins and to fathom how they prevent the formation of new memories,” says Susswein,

“We may also find that the blocking process accounts for why we cannot remember our dreams when we wake up.”

One exciting possibility is that is these proteins can inhibit memory formation, they could potentially be used to block unwanted or traumatic memories such as those of PTSD patients.

The full paper “New learning while consolidating memory during sleep is actively blocked by a protein synthesis dependent process” has been published in the journal eLife.